The objective of our research is to investigate the role of free radicals in biological electron transport and to probe the mechanism by which electrons are transferred from the exterior of an enzyme to its active site. This goal will be pursued by employing a series of small molecule model studies. Specifically, we propose to investigate the reactions of a group of biologically related transition metal complexes with a variety of free radicals. The model complexes will be analogues to those known to constitute the active sites of B12-dependent enzymes, cytochrome c and cytochome c oxidase. The free radicals will represent analogues of the amino acid side chains which are present in the interior of the enzymes. These radicals will be generated either thermally or photolytically, depending upon the stability of the particular system under investigation. The mechanisms of reactions between complexes and free radicals will be deduced by (a) analysis of products, (b) competition reactions, (c) trapping of intermediates, and (d) ESR spectroscopic and electrochemical methods. The ultimate aim of this research is the extension of our findings to the biological system.